1. Field of the Invention
The present invention relates to elastic wave devices including a plurality of longitudinally coupled resonator elastic wave elements, and duplexers as well as multiplexers including the elastic wave devices.
2. Description of the Related Art
Various types of filter devices and the like using longitudinally coupled resonator elastic wave elements have been proposed. In an elastic wave device disclosed in International Publication No. WO 2009/150786, a first longitudinally coupled resonator elastic wave filter and a second longitudinally coupled resonator elastic wave filter are disposed on a piezoelectric substrate. The first longitudinally coupled resonator elastic wave filter and the second longitudinally coupled resonator elastic wave filter are aligned so that elastic wave propagation regions thereof overlap with each other when viewed in an elastic wave propagation direction. Further, the first longitudinally coupled resonator elastic wave filter and the second longitudinally coupled resonator elastic wave filter are connected in parallel. Furthermore, a ground interconnection is provided in a region between reflectors at a side where the first and second longitudinally coupled resonator elastic wave filters are adjacent to each other.
The elastic wave device disclosed in International Publication No. WO 2009/150786 is also required to be small in size like other electronic components. To meet this requirement, it is necessary to shorten a distance between the reflectors adjacent to each other of the first and second longitudinally coupled resonator elastic wave filters.
Meanwhile, because waves cannot be completely confined in the longitudinally coupled resonator elastic wave filter, the waves leak to an outer side portion in the elastic wave propagation direction beyond the reflector in some cases. As such, the waves leaked from the first longitudinally coupled resonator elastic wave filter toward the second longitudinally coupled resonator elastic wave filter side interfere with the waves leaked from the second longitudinally coupled resonator elastic wave filter toward the first longitudinally coupled resonator elastic wave filter side. In this case, as a distance between the first and second resonator elastic wave filters is shorter, influence of the interference of the waves as unwanted modes becomes larger.
The reflector can be designed to prevent the generation of an unwanted mode, that is, a high order longitudinal mode positioned on a lower frequency side of the pass band, for example. In such case, however, it is difficult to obtain steepness on the lower frequency side of the pass band.